This paper describes the equivalent homogeneous uniaxial mechanical properties of defective single-wall carbon nanotubes. In particular, non reconstructed defects that can be produced by ion or electronic irradiation have been considered. A discrete nonlinear finite-element approach based on the mechanical properties of individual carbon carbon (C–C) bonds has been used. The individual C–C bonds in turn were simulated as beam structural elements. Extensive Monte Carlo based numerical simulation has been reported in the paper. The results show that the homogeneous elastic properties of the defective nanotubes can be qualitatively and quantitatively different from the pristine configurations. The defective nanotubes show a slight reduction in axial stiffness (Young’s modulus), but large variations of Poisson’s ratio outside the elastic bounds for isotropic materials, depending on the locations of the vacancies. The large fluctuations of Poisson’s ratio can lead to extreme positive transversal contractions or to auxetic behaviour when the nanotubes are subjected to tensile loading.